Enzymes that use NAD+ play a part in the DNA repair process. Specifically, the poly(ADP-ribose) polymerases (PARPs), particularly PARP-1, are activated by DNA strand breaks and affect DNA repair. The PARPs consume NAD+ as an adenosine diphosphate ribose (ADPR) donor and synthesize poly(ADP-ribose) onto nuclear proteins such as histones and PARP itself. Although PARP activities facilitate DNA repair, overactivation of PARP can cause significant depletion of cellular NAD+, leading to cellular necrosis. The apparent sensitivity of NAD+ metabolism to genotoxicity has led to pharmacological investigations into the inhibition of PARP as a means to improve cell survival. Numerous reports have shown that PARP inhibition increases NAD+ concentrations in cells subject to genotoxicity, with a resulting decrease in cellular necrosis. Nevertheless, cell death from toxicity still occurs, presumably because cells are able to complete apoptotic pathways that are activated by genotoxicity. Thus, significant cell death is still a consequence of DNA/macromolecule damage, even with inhibition of PARP. This consequence suggests that improvement of NAD+ metabolism in genotoxicity can be partially effective in improving cell survival but that other players that modulate apoptotic sensitivity, such as sirtuins, may also play important roles in cell responses to genotoxins.
Physiological and biochemical mechanisms that determine the effects of chemical and radiation toxicity in tissues are complex, and evidence indicates that NAD+ metabolism is an important player in cell stress response pathways. For example, upregulation of NAD+ metabolism, via nicotinamide/nicotinic acid mononucleotide overexpression, has been shown to protect against neuron axonal degeneration, and nicotinamide used pharmacologically has been recently shown to provide neuron protection in a model of fetal alcohol syndrome and fetal ischemia. Such protective effects could be attributable to upregulated NAD+ biosynthesis, which increases the available NAD+ pool subject to depletion during genotoxic stress. This depletion of NAD+ is mediated by PARP enzymes, which are activated by DNA damage and can deplete cellular NAD+, leading to necrotic death. Another mechanism of enhanced cell protection that could act in concert with upregulated NAD+ biosynthesis is the activation of cell protection transcriptional programs regulated by sirtuin enzymes.
Examples of cell and tissue protection linked to NAD+ and sirtuins include the finding that SIRT1 is required for neuroprotection associated with trauma and genotoxicity. SIRT1 can also decrease microglia-dependent toxicity of amyloid-beta through reduced NFKB signaling. SIRT1 and increased NAD+ concentrations provide neuroprotection in a model of Alzheimer's disease. Sirtuins are NAD+-dependent enzymes that have protein deacetylase and ADP-ribosyltransferase activities that upregulate stress response pathways. Evidence indicates that SIRT1 is upregulated by calorie restriction and in humans could provide cells with protection against apoptosis via downregulation of p53 and Ku70 functions. In addition, SIRT1 upregulates FOXO-dependent transcription of proteins involved in reactive oxygen species (ROS) detoxification, such as MnSOD. The sirtuin SIRT6 has been shown to participate in DNA repair pathways and to help maintain genome stability. With respect to nicotinyl ribosides including nicotinamide riboside, various uses have been proposed as in U.S. Pat. No. 8,106,184, herein incorporated by reference.
UV-Mediated DNA Damage
Ultraviolet (UV) light plays an integral role in the development of numerous skin ailments ranging from aging to cancer. Considerable evidence spanning decades has conclusively demonstrated that UV radiation triggers multiple independent cellular responses. UV radiation is known to penetrate skin where it is absorbed by proteins, lipids and DNA, causing a series of events that result in progressive deterioration of the cellular structure and function of cells (Valacchi, et al., “Cutaneous responses to environmental stressors,” Ann. N. Y. Acad. Sci. (2012) 1271: 75-81). DNA is the building block of life and its stability is of the utmost importance for the proper functioning of all living cells. UV radiation is one of the most powerful (and common) environmental factors that can cause a wide range of cellular disorders by inducing mutagenic and cytotoxic DNA lesions; most notably cyclobutane-pyrimidine dimers (CPDs) and 6-4 photoproducts (64 pps) (Narayanan, et al., “Ultraviolet radiation and skin cancer,” Int. J. Dermatol. (2010) 49: 978-86). It is important to note that UV-mediated DNA damage is an early event in a plethora of proliferative cellular disorders. The two major types of UV-induced DNA damage are CPDs and 64 pp (along with their Dewer isomers) (Sinha, R. P. and Hader, D. P., “UV-induced DNA damage and repair: a review,” Photochem. Photobiol. Sci. (2002) 1: 225-36; and Rastogi, et al., “Molecular mechanisms of ultraviolet radiation-induced DNA damage and repair,” J. Nucleic Acids (2010) 2010: 592980). These abundant DNA lesions, if unrepaired, can interfere with DNA replication and subsequently cause mutations in DNA. Thus, these lesions can be mutagenic (potentially leading to proliferative disorders) and/or can be cytotoxic (resulting in cell death). 64 pp occur at about one third the frequency of CPDs, but are more mutagenic (Sinha & Hader, 2002). In one embodiment, prevention of these UV-mediated DNA adducts is paramount to guarding against the onset of several proliferative disorders, ranging from aging to cancer.
UV-Mediated Loss of Barrier Function
Maintaining a water-impermeable barrier between the organism and the environment is an essential function of skin. This barrier function serves to prevent dehydration; which can lead to death of the organism (Jiang, S. J., et al., “Ultraviolet B-induced alterations of the skin barrier and epidermal calcium gradient,” Exp. Dermatol. (2007) 16: 985-992). UV light has been demonstrated to disrupt epidermal skin barrier function in a dose-dependent manner (Haratake, A., et al., “UVB-induced alterations in permeability barrier function: roles for epidermal hyperproliferation and thymocyte-mediated response” J. Invest. Dermatol. (1997) 108: 769-775; and prev. citation). Skin barrier dysfunction can be directly assessed by measuring Transepidermal Water Loss (TEWL), which is a measure of skin hydration (Oba, C., et al., “Collagen hydrolysate intake improves the loss of epidermal barrier function and skin elasticity induced by UVB irradiation in hairless mice,” Photodermatol. Photoimmunol. Photomed. (2013) 29: 204-11; and prev. citations).
Therefore, it is hypothesized that a chemopreventative agent for several human skin disorders will be effective at inhibiting or preventing the direct UV-mediated loss of barrier function, DNA damage, or oxidative damage in helping to maintain healthy human skin.
If a way could be found to use nicotinamide riboside, or salts thereof, in a topical skin care composition in the maintenance of healthy human skin, this would represent a useful contribution to the art. Furthermore, if a way could be found to use nicotinamide riboside, or salts thereof, in a cosmetic or cosmeceutical composition in the maintenance of healthy human skin, this would also represent a useful contribution to the art.